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Os 91 fs

Old 01-26-2014, 12:00 PM
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Exclamation Os 91 fs

I have a 91 FS ( non pump version) that will not run right, it does not seem to follow
needle adjustments in that you move the needle and it does not make any difference
than all of a sudden , it seems to decide to either richen or lean out. In the air if
you back off the throttle for the down leg of the loop when you re apply it, it does nothing,
you fly off then you get a burst of throttle, and quite offen it will refuse to rev up then quits.
This can happen also just flying around level at half throttle, as soon as you apply throttle
it does not respond then will quit.
Any help much appreciated.
Old 01-27-2014, 07:24 AM
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First, please check the fuel system very carefully to make sure it's free-flowing and there is no debris or any kind of restriction. Check it for air leaks anywhere. Then check the carburetor. Make sure it's metering correctly. Many times, the high-speed needle has damaged the needle seat in the carburetor body, and it won't meter well. You can tell this by the fact that the needle can be almost completely closed without causing the engine to quit.

Make sure the valves are seating properly and the rockers are properly-set. Make sure you have good compression. A bad ring to liner fit will cause many operating problems, especially problems at idle and at throttle-up.

A lot of things to check, and that's how engines are. You need to go through them carefully. Working with an engine test stand is a big help.

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